Learn Hardcore Japanese: watasi ha sono mise de yasui mono wo kaimasita.

は is the topic marker: it tells the listener “we’re talking about me.” It sets 私 as the topic of the sentence. が, on the other hand, marks the subject, often introducing new information. In this sentence, you’re not introducing 私 for the first time; you’re stating what “I” did. Therefore, 私は (“as for me”) is appropriate.

What role does で play in 店で?

で marks the location where an action takes place. Here, その店で means “at that shop.” It tells us the place where the buying happened. If you used に instead, 店に would more often indicate a destination (“to the shop”) or existence, not the site of the action.

Why is その used before 店? Could we use あの or この instead?

その means “that (near you/on the same topic),” referring to a shop already mentioned or visible to the listener.

この店 = “this shop” (near the speaker).
あの店 = “that shop (over there)” (far from both speaker and listener).
Choose based on which shop you’re referring to in the conversation.

What does 安い物 mean, and why not just 安い?

安い物 literally means “cheap things” or “inexpensive items.”

安い by itself is an adjective (“cheap”), not a noun.
To talk about a “cheap object,” you need a noun after the adjective. 物 (もの) is a general noun meaning “thing” or “object.” Without 物, the sentence would be incomplete.

Why is 物 pronounced もの here, and are there other readings?

物 has two main readings:

Kun‐reading もの (“thing, object”).
On-reading ぶつ (used in compounds like 食べ物 [たべもの] = “food,” or 荷物 [にもつ] = “luggage”).
In standalone usage meaning “a thing,” you always use the kun-reading もの.

What is the function of the particle を in 物を買いました?

を marks the direct object of a verb. Here, 物 is what was bought, so 物を tells us “the thing” is the object of 買いました (“bought”).

What does 買いました indicate in terms of tense and politeness? Could I say 買った instead?

買いました is the polite past form of 買う (“to buy”), indicating a completed action (“bought”).

買った is the plain (dictionary-style) past: still past tense but less formal.
Use 買いました in polite or business contexts; use 買った in casual conversation.

Is the word order strictly fixed as 私はその店で安い物を買いました?

Japanese is relatively flexible, but the verb almost always comes at the end. The usual order for a simple sentence is:

Topic (私は)
Time (if any)
Place (その店で)
Object (安い物を)
Verb (買いました)

You can rearrange topics or add emphasis (e.g., その店では私は安い物を買いました to emphasize その店で), but the verb remains at the end and particles stay attached to their words.